The lipid bilayer is a semipermeable, incompressible, two dimensional liquid crystal which is asymmetric in composition and separates the cytoplasm, within the cell, from the extracellular medium. Phosphatidylcholine (PC), sphingomyelin and the sterol cholesterol are the dominant extra leaflet components while phosphatidylserine (PS) and phosphatidylethanolamine (PE) are the dominant inner leaflet components. Interestingly enough there are proteins in the bilayer called flippases which maintain the correct lipid asymmetry.
Transmembrane proteins are solutes in a two dimensional fluid, the bilayer, and thus have varying degrees of lateral mobility in the plane of the membrane. Dynamics. They have a transbilayer domain and either or both an extrafacial domain, which contributes to the glycocalyx and a cytoplasmic domain. The major transmembrane proteins are glygoproteins, band 3 and glygophorin. Band 3 is a multispanning ion transport channel and exists in a dimer / tetramer equilibrium. It is structurally important because band 3 tetramers, rather than dimers, tether the bilayer to the skeleton via an interaction between its cytoplasmic domain and ankyrin which is associated with spectrin. Glycophorins have a single spanning alpha helix and are a general class of proteins which contribute the major portion of glycosylation (sugar) at the extracellular domain. Glycophorin C (GPC) is another bilayer / skeleton tethering point via its interaction with protein 4.1 within the junctional complex. Glycophorin A (GPA) is partially associated with band 3. There are a host of other transmembrane proteins. Aquaporin, as the name suggests, is a water channel protein existing as a homotetramer. Rh is a protein complex thought to associate with band 3. One class of proteins, which are not strictly transbilayer, are Glycosylphosphatidylinositol (GPI)-linked proteins. GPI is a lipid analog which links an extrafacial protein, for example, CD59.
The cytoskeleton is an irregular hexagonal lattice of polymeric spectrin molecules which are tied together by actin, 4.1 and other numerous proteins at nodes called junctional complexes. The skeleton makes a two dimensional network which is very flexible, compressible with obvious structural importance.